A tankless water heater size chart is only worth reading if it has two axes, and almost every one you will find has one. It lists gallons per minute against a number of bathrooms, which quietly assumes that every house in the country takes its water in at the same temperature. That assumption is how people end up with a unit that was fine in September and disappoints them in February.
The capacity of a tankless unit is set by two things multiplied together: how much water is moving at once, and how far that water has to be lifted in temperature. Flow is the easy half, and you can count it off your own fixtures in about a minute. Rise is the half that moves the answer, because the main comes in at roughly 75 °F in south Florida and roughly 45 °F in Minnesota, and the same appliance behaves like two different appliances in those two houses. Both charts are below. If you are sizing a storage tank instead, the capacity worksheet for tanks answers a different question and you want that one.
The short answer
- One bathroom, warm climate (Gulf coast, Florida, south Texas): 3.5 to 4 GPM covers you, and at a 45 °F rise that is 83,000 to 95,000 BTU/hr of input.
- Two bathrooms, moderate climate (mid-Atlantic, Carolinas, most of California): plan on 5 to 5.5 GPM at a 60 °F rise, which lands at 158,000 to 174,000 BTU/hr of input.
- Two or more bathrooms, cold climate (upper Midwest, northern New England, mountain states): you are at the ceiling of a single residential unit at about 5 GPM. Anything more and you are buying two units.
- One remote fixture — a workshop sink, a bathroom at the far end of a ranch: point-of-use electric, sized for that fixture alone, not a whole-house unit.
- Anyone shopping on the GPM number printed on the carton: that figure is quoted at a temperature rise most of the country never sees. Find the flow-versus-rise table in the spec sheet and read your own band off it.
Two numbers, multiplied
Every tankless unit ever built is sized by one line of arithmetic. BTU per hour equals gallons per minute times temperature rise times 500. The 500 is just water and clock: a gallon weighs 8.34 pounds, one BTU lifts one pound of water one degree, and there are 60 minutes in an hour, so 8.34 × 60 = 500 and change. That is the entire chart. Everything below is that line with your numbers in it.
Electric units are quoted in kilowatts, and the same physics reads kW = GPM × rise × 0.1465. Five gallons a minute at a 75 °F rise is 54.9 kW. Almost no existing American house has that much electrical service sitting spare, which is why whole-house electric tankless is a service question long before it is an efficiency question. I have written that out properly in the electric versus gas tankless comparison. Sizing the circuit is a job for a licensed electrician, and this site does not do it.
Three rise bands cover most of the country if you want 120 °F at the tap. The warm band is a 45 °F rise: Florida, the Gulf coast, south Texas, Hawaii, incoming around 75 °F. The moderate band is a 60 °F rise: the mid-Atlantic, the Carolinas, Tennessee, most of California, the coastal Northwest, incoming around 60 °F. The cold band is a 75 °F rise: Minnesota, the Dakotas, northern New England, the mountain states, incoming around 45 °F in the depth of winter. Pick your band on your coldest month rather than your annual average. The coldest month is the one that generates the complaint.
Chart one: what your fixtures actually draw
Flow ratings are printed on the fixture, stamped into the aerator, or listed in the appliance manual. Where they are not, these are the numbers the trade works from. Add up only the fixtures that genuinely run at the same moment, which for most houses is a shower plus one sink, not the whole plumbing schedule.
| fixture | TYPICALstandard fitting, GPM | LOW-FLOWWaterSense or better, GPM |
|---|---|---|
| Shower head | 2.5federal maximum since 1992 | 1.75saves 0.75 GPM per showersets the whole answer |
| Bathroom faucet | 1.5rarely run hot for long | 1.0common on newer fittings |
| Kitchen faucet | 2.2pull-down sprayers run high | 1.5aerator swap territory |
| Dishwasher | 1.0many models heat their own | 0.9short, intermittent drawsmallest load |
| Clothes washer | 2.0top loader, warm cycle | 1.5front loader; cold cycles draw none |
| Tub fill | 4.0tub spouts are not restrictedlargest single draw | 4.0no low-flow version exists |
One shower or two? That single question moves the required unit by roughly 60,000 BTU/hr. Filling a tub during the morning rush? Add roughly three gallons a minute of hot draw and read the next chart again.
Chart two: side by side, capacity by climate band
This is the chart the keyword is really asking for. Find your simultaneous draw in the left column, then read across to your rise band. Carry one thing over from the first chart: the draws below are hot-water flow, not total flow at the fixture. A shower valve blends cold in, so a 2.5 GPM head running at a comfortable 105 °F pulls about 2.0 GPM off the hot side when the main is cold. Sinks work the same way. The figures are input capacity, which is the number manufacturers print on the unit, calculated as delivered heat divided by 0.95 for a condensing burner. A non-condensing unit runs closer to 0.83, so add about fifteen percent to each cell if that is what you are buying. The condensing comparison covers when that difference is worth paying for.
| simultaneous draw | WARM45 °F rise, BTU/hr in | MODERATE60 °F rise, BTU/hr in | COLD75 °F rise, BTU/hr in |
|---|---|---|---|
| One shower — 2.0 GPM | 47,000entry-level uniteasiest case | 63,000small unit still fine | 79,000smallest whole-house class |
| Shower + bathroom sink — 2.5 GPM | 59,000comfortable margin | 79,000common one-bath answer | 99,000mid-size unit |
| Shower + kitchen — 3.5 GPM | 83,000mid-size unit | 111,000mid-size unit | 138,000large unit |
| Two showers — 4.0 GPM | 95,000mid-size unit | 126,000large unit | 158,000near the top of the range |
| Two showers + kitchen — 5.5 GPM | 130,000large unit | 174,000top of the residential rangenear the ceiling | 217,000past one unittwo units |
| Two showers + kitchen + laundry — 7.0 GPM | 166,000top of the residential range | 221,000past one unittwo units | 276,000well past one unittwo units |
Warm band? One condensing unit covers nearly any single-family house on this chart. Cold band? Two simultaneous showers plus a running kitchen tap is already more than one residential gas unit can deliver, and no brand changes that.
Reading it off for one real house
A family of four outside Columbus, Ohio. Two teenagers shower back to back at seven, except on the mornings when they do not, and somebody is at the kitchen sink. Two 2.5 GPM shower heads pull about 2.0 GPM of hot water each, and a kitchen tap run warm adds about another gallon a minute. Call it 5.0 GPM on the hot side. Municipal water arrives at about 55 °F in January, so the rise to 120 °F is 65 °F, sitting between the moderate and cold bands.
Delivered heat: 5.0 × 65 × 500 = 162,500 BTU/hr. Divide by 0.95 for a condensing burner and the unit needs 171,000 BTU/hr of input. That points at the 180,000 to 199,000 BTU/hr class, which is the top of the residential range. Run it the other way to check the margin: the largest common residential units are rated 199,000 BTU/hr, so 199,000 × 0.95 = 189,050 BTU/hr delivered, and 189,050 ÷ (65 × 500) = 5.8 GPM. Eight tenths of a gallon a minute in hand. Enough, not generous.
Move the identical family to Minneapolis, where the main is around 45 °F in February. Same fixtures, same 5.0 GPM, but the rise is now 75 °F. That is 5.0 × 75 × 500 = 187,500 BTU/hr delivered, or 197,400 of input, which is the entire capacity of the biggest unit on the shelf with nothing left over. Same household, same plumbing, different state, different answer. This is why bathroom-count charts fail.
The number on the box is quoted at a rise you do not have
Here is the concession that undoes half of what I have just written. Manufacturers advertise a headline GPM figure, and that figure is measured at a temperature rise chosen to flatter the unit, frequently 35 °F or 45 °F. A unit sold as “11 GPM” is an 11 GPM unit somewhere warm. In the cold band it is a 5 GPM unit, and the carton does not say so on the front.
Every serious manufacturer publishes a flow-versus-rise table in the specification sheet, usually a short grid running from a 30 °F rise to an 80 °F rise. That grid is the honest version of the marketing number, and it takes thirty seconds to find. Read your band off it and ignore the front of the box entirely. The AHRI directory lists certified input rating and UEF for the same models if you want a second source that has no interest in selling you anything.
The other thing no chart can tell you is your own incoming water temperature. Well water and municipal water differ, a shallow main under a hot street differs from a deep one, and the seasonal swing in one house can be twenty degrees. A thermometer under a running cold tap in the coldest week of the year settles it for nothing. Until then, treat your band as an estimate and lean cold.
Buying a size up costs once, not every day
Oversizing a storage tank is a running penalty, because a bigger cylinder has more surface area and bleeds standby heat every hour of its life. A tankless unit has no standby loss worth arguing about. It fires for the water you draw, so a 199,000 BTU/hr unit heating a modest household burns the same fuel as a 150,000 BTU/hr one heating that household. Capacity you never use is a one-time cost, not a recurring one.
Put a number on the fuel side so the comparison is concrete. The standard household load on this site is 4,000 kWh a year of delivered heat, which is 136 therms. Through a condensing unit at UEF 0.95 that is 136 ÷ 0.95 = 143 therms of gas, or $215 a year at $1.50 a therm, roughly $2,150 over ten years. Through a non-condensing unit at 0.83 it is 164 therms, $246 a year, about $2,460 over ten years. Neither figure moves because you bought the larger burner. Equipment for gas tankless runs $1,000 to $2,000, and stepping up one capacity class within a manufacturer’s line is usually a few hundred dollars of that spread.
So my judgement, owned: when the calculated figure lands between two models, I take the larger one. The downside is a few hundred dollars once. The downside of the other choice is a cold rinse every winter morning for fifteen years. Where I would not go up a size is the household that genuinely never runs two hot fixtures together, which is more households than the charts admit. A retired couple in a two-bathroom house is not a two-shower load, and buying for a peak that never arrives is money spent on a scenario. If you are unsure which of those you are, the longer tankless sizing walkthrough works a household through it properly. Once you know the capacity class, the brand question narrows fast, and Rinnai against Rheem is where most people land.
Sources
- DOE Energy Saver, guidance on sizing demand (tankless) water heaters by flow rate and temperature rise.
- AHRI Directory of Certified Product Performance, for certified input rating and uniform energy factor by model.
- ENERGY STAR certified product finder, gas tankless water heaters, for the current certified capacity range.
- FTC EnergyGuide label, for the annual energy figure printed on the unit.
- Manufacturer specification sheets, specifically the flow-versus-temperature-rise tables published by Rinnai, Navien and Rheem.
- US Energy Information Administration, average residential natural gas and electricity prices.
A size chart is a starting point and not a specification. It gets you to a capacity class in five minutes, and then the manufacturer’s own flow-versus-rise grid and a thermometer under your cold tap finish the job. Everything above uses the same assumptions every cost model on this site uses, and they are all written down on the method page so you can disagree with them in an informed way.